To date, various surface mount inductors have been proposed in order to form compact power supply circuits. For example, in Patent Document 1, an inductor is disclosed that has an external connection terminal formed at each of the two opposing ends of a rectangular-parallelepiped-shaped multilayer body. An inductor composed of a spiral-shaped conductor is formed inside the multilayer body. One end of the inductor is connected to one of the external connection terminals and the other end of the inductor is connected to the other external connection terminal.
FIG. 9 is an exploded perspective view of a multilayer inductor 100P of the related art described in Patent Document 1. FIG. 10 is a sectional view of the multilayer inductor 100P of the related art. In FIG. 9, illustration of external connection terminals 171P and 172P is omitted. FIG. 10 is a sectional view looking at a plane orthogonal to end surfaces on which the external connection terminals 171P and 172P are formed.
The multilayer inductor 100P includes a rectangular-parallelepiped-shaped multilayer body formed by stacking flat-plate-shaped magnetic layers 101P to 106P in a direction orthogonal to the surfaces of the layers, and the external connection conductors 171P and 172P that are each formed on one of the two ends of the multilayer body located in a direction orthogonal to the stacking direction.
Winding line-shaped conductors 121P, 122P, 123P, 124P and 125P are respectively formed on the five magnetic layers 102P, 103P, 104P, 105P and 106P. The line-shaped conductors 121P, 122P, 123P, 124P and 125P are connected to one another in the stacking direction by interlayer connection conductors 141P, 142P, 143P and 144P. With this configuration, a spiral-shaped inductor having an axis that extends in the stacking direction is formed. One end of the line-shaped conductor 121P, which forms one end of the inductor, is exposed at an end surface of the multilayer body and is connected to the external connection conductor 172P. The other end of the line-shaped conductor 125P, which forms the other end of the inductor, is exposed at the other end surface of the multilayer body and is connected to the external connection conductor 171P.
The external connection conductors 171P and 172P are formed on not only opposing end surfaces of the multilayer body but rather are formed in such a shape as to also extend onto a top surface, a bottom surface and two side surfaces of the multilayer body.
When mounting the multilayer inductor 100P having the above-described form, the external connection terminals 171P and 172P are arranged on and bonded with solder to mounting lands.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2010-165964
FIG. 11 is a diagram illustrating a mounting configuration of a power supply circuit module including the multilayer inductor 100P of the related art. The power supply circuit module is realized by mounting the multilayer inductor 100P, capacitors 211 and 212 and a switch IC element 201 on a front surface of a base circuit board 200.
Here, in the case of the multilayer inductor 100P, which has the external connection conductors 171P and 172P as described above, in order to secure bonding reliability, as illustrated in FIG. 11, it is necessary for solder fillets to extend over the end, side and bottom surfaces of the external connection conductors 171P and 172P. At this time, the solder sometimes also spreads onto the top surface.
Consequently, as illustrated in FIG. 11, the mounting lands have to be formed so as to extend beyond a region corresponding to the area of the multilayer inductor 100P on the mounting surface, and the area dedicated to mounting of the multilayer inductor 100P is increased.
In addition, the surface of the board 200 on which the individual elements, including the multilayer inductor 100P, are mounted is generally covered with a shield member 220, which realizes electromagnetic shielding. However, since the shield member 220 is composed of a conductive material, top-surface-side portions of the external connection conductors 171P and 172P of the multilayer inductor 100P and solder that has spread onto these top-surface-side portions may come into contact with the shield member 220 and cause short circuit failures to occur. Therefore, the shield member 220 has to be formed and arranged such that a gap Gp, which is of such a size that shorts due to for example variations in the manufacturing process do not occur, is provided between the top surface of the multilayer inductor 100P and a top plate of the shield member 220 and this leads to an increase in the profile of the power supply circuit module.
Consequently, a multilayer inductor 100PP has been considered that has a structure in which the external connection conductors 171P and 172P are not formed on the end surfaces, and in which, as illustrated in FIG. 12, external connection conductors 161PP and 162PP are formed on a bottom surface of the multilayer body. FIG. 12 is an exploded perspective view of the typical LGA type multilayer inductor 100PP.
The multilayer inductor 100PP includes a rectangular-parallelepiped-shaped multilayer body obtained by stacking flat-plate-shaped magnetic layers 101PP to 107PP in a direction orthogonal to the surfaces of the layers.
Winding line-shaped conductors 121PP, 122PP, 123PP, 124PP and 125PP are formed on the five magnetic layers 102PP, 103PP, 104PP, 105PP and 106PP. The line-shaped conductors 121PP, 122PP, 123PP, 124PP and 125PP are connected to one another in the stacking direction by interlayer connection conductors 141PP, 142PP, 143PP and 144PP. With this configuration, a spiral-shaped inductor having an axis that extends in the stacking direction is formed.
One end of the line-shaped conductor 125PP, which is a lowermost-layer-side end portion of the inductor in the stacking direction, is connected to the external connection conductor 161PP on the bottom surface of the multilayer body via an interlayer connection conductor 154PP.
Another end of the line-shaped conductor 121PP, which is an uppermost-layer-side end portion of the inductor in the stacking direction, is connected to a line-shaped conductor 131PP formed on the magnetic layer 102PP, on which the line-shaped conductor 121PP is formed. The line-shaped conductor 131PP is formed in such a shape as to extend toward the inside from the winding line-shaped conductor 121PP.
The line-shaped conductor 131PP is connected to a line-shaped conductor 132PP formed on the magnetic layer 107PP via an interlayer connection conductor 150PP, which penetrates through the magnetic layers 102PP, 103PP, 104PP, 105PP and 106PP. The line-shaped conductor 132PP is connected to the external connection conductor 162PP on the bottom surface of the multilayer body via an interlayer connection conductor 153PP.
Since the mounting lands are below the bottom surface of the multilayer inductor 100PP as a result of using the LGA type multilayer inductor 100PP having the external connection conductors 161PP and 162PP formed on the bottom surface in this way, the area dedicated to mounting can be reduced. In addition, the top surface of the multilayer inductor 100PP has an insulation property and therefore even if it contacts the shield member there is no problem and it is possible to reduce the profile of the power supply circuit module.
However, there is the following problem with the LGA type multilayer inductor 100PP having the structure illustrated in FIG. 12. FIG. 13 shows diagrams for explaining a problem in a case where the typical LGA type multilayer inductor 100PP is used. FIG. 13(A) is a sectional view taken along cross section A-A′ in FIG. 12. FIG. 13(B) is a sectional view taken along cross section B-B′ in FIG. 12.
In the typical LGA type multilayer inductor 100PP, the line-shaped conductor 131PP, which is for routing the uppermost-layer end portion of the inductor to the external connection conductor 162PP on the bottom surface of the multilayer body, is on the same layer as the line-shaped conductor 121PP of the inductor of the multilayer inductor 100PP, and therefore, as illustrated in FIG. 13(A), the line-shaped conductor 131PP disturbs formation of magnetic flux by the inductor composed of the line-shaped conductors 121PP to 125PP. As a result of this, various characteristics of the inductor are degraded.